The present invention relates to free electron lasers.
The following references are cited in this background to better illustrate the background of the invention:
Ref. 1 U.S. Pat. No. 3,822,410 issued to Madey for xe2x80x9cStimulated Emission of Radiation in a Periodically Deflected Electron Beamxe2x80x9d;
Ref. 2 J. M. J. Madey, xe2x80x9cStimulated Emission of Bremsstrahlung in a Periodic Magnetic Field,xe2x80x9d J. Appl. Phys., vol. 42, pp. 1906-1913, 1971;
Ref. 3 J. M. J. Madey, D. A. G. Deacon, L. R. Elias, and T. I. Smith, xe2x80x9cAn Approximate Technique for the Integration of the Equations of Motion in a Free-Electron Laser,xe2x80x9d Il Nuovo Cimento, vol. 51B, pp. 53-69, 1979;
Ref. 4 W. B. Colson, Free Electron Laser Theory, Ph.D. Dissertation, Stanford, Calif.: Department of Physics, Stanford University, 1977;
Ref. 5 N. M. Kroll, P. L. Morton, and M. N. Rosenbluth, xe2x80x9cFree-Electron Lasers with Variable Parameter Wigglers,xe2x80x9d IEEE J. Quantum Electron., vol. QE-17, pp. 1436-1468, 1981;
Ref. 6 ibid;
Ref. 7 C. A. Brau, Free-Electron Lasers, Boston, Mass.: Academic Press, 1990; pp. 236-255;
Ref. 8 op. cit., ref. 3;
Ref. 9 op. cit., ref. 5;
Ref. 10 op. cit., ref. 7; pp. 255-258;
Ref. 11 O. K. Crisafulli, E. B. Szarmes, and J. M. J. Madey, xe2x80x9cUse of Inverse-Tapering to Optimize Efficiency and Suppress Energy Spread in an rf-Linac Free-Electron Laser Oscillator,xe2x80x9d IEEE J. Quantum Electron., vol. 37, pp.993-1007, 2001;
Ref. 12 U.S. Pat. No. 4,641,103 issued to Madey et al. for xe2x80x9cMicrowave Electron Gunxe2x80x9d; and
Ref. 13 U.S. Pat. No. 5,130,994 issued to Madey et al. for xe2x80x9cFree-electron laser oscillator for simultaneous narrow spectral resolution and fast-time resolution spectroscopy.xe2x80x9d.
A free-electron laser is a device used to convert the kinetic energy of a beam of relativistic free electrons to electromagnetic radiation in the wavelength region between the mm-wave region in radio spectrum and the extreme ultraviolet region at optical wavelengths (Ref. 1). As is known in the art, free-electron lasers work by exploiting the interaction between a beam of relativistic electrons moving through a spatially oscillating transverse magnetic field and a co-propagating beam of electromagnetic radiation (Ref. 2).
In the presence of a strong optical field whose phase matches the phase of the transverse oscillations induced by the transverse magnetic field, the electrons"" trajectories in phase and energy are governed by a pair of coupled equations which can be reduced to the pendulum equation (Ref. 3; Ref. 4). The existence of a series of fixed points and an associated set of stable, closed, periodic orbits (xe2x80x9cbucketsxe2x80x9d) in the phase space trajectories followed by the electrons in such a system has led to the development of a range of methods for enhancement of the power output that can be obtained from such devices (Ref. 5).
The principal method for enhancement of free-electron laser power output and efficiency employed to date has been the deceleration of the electrons circulating in the periodic orbits around the stable fixed points in their phase space trajectories by decreasing the period and/or amplitude of the spatially oscillating magnetic field as a function of position along the interaction region (Ref. 6; Ref. 7). This method has yielded only limited improvements in power output, and has the further disadvantage of failing to extract a significant amount of energy from the electrons moving along the open trajectories outside the region of phase stability, leaving the electrons emerging from the interaction distributed over a range of energies as large as 10% or more. Independent of the limited enhancement in power output attainable by this method, the large energy spread induced by this method has complicated attempts to recover the residual kinetic energy of the spent electrons, and made it impossible to operate more than one free-electron laser at a time using a single beam of electrons.
The expense of the accelerator systems required to produce the electron beams required for free-electron laser operation and the intense ionizing radiation produced by the spent electrons emerging from the interaction region if not decelerated have constituted major practical barriers to the further development and commercialization of free-electron lasers. The invention described herein overcomes these barriers by exploiting a different lasing mechanism, phase displacement (Ref. 8; Ref. 9; Ref. 10), in which the kinetic energy of electrons moving through an interaction region defined by a spatially oscillating transverse magnetic field and a co-propagating beam of electromagnetic radiation is converted to light by facilitating the deceleration of the electrons to lower energies along the open trajectories outside and between the regions of phase stability (xe2x80x9cbucketsxe2x80x9d). By permitting deceleration of nearly all the electrons in the beam by nearly the same increment, this method provides greater laser power output than previously attainable by converting a greater fraction of the electrons average energy to electromagnetic radiation, while dramatically reducing the spread in energy induced by the interaction thereby simplifying the transport and recovery of the residual kinetic energy of the spent electron beam and reducing the risk of production of hazardous ionizing radiation during operation.
Accordingly, the invention described herein provides a means to enhance the power output, efficiency and flexibility of free-electron lasers while reducing their cost and complexity and the cost of the ancillary radiation shielding required to insure operator and public safety. These improvements are important to currently established and existing uses for free-electron lasers including laser surgery and diagnostics, materials processing, spectroscopy and remote sensing, laser power beaming, and high power laser weapons systems.
Although certain aspects of the phase displacement lasing mechanism were anticipated in the earlier publications, the inventors were the first to rigorously examine the operation of systems based on this mechanism under the conditions prevailing in practical use in which lasers must start from noise in the small signal regime, evolve naturally to achieve the conditions required for effective conversion of electron kinetic energy to light, and stably maintain these parameters for a useful interval of time (Ref. 11). In the course of this new and unanticipated investigation, the inventors were able to establish that such systems, properly designed, could start from noise and evolve naturally to a stable operating configuration compatible with enhanced power output and reduced output energy spread. However, the inventors have also discovered a new instability capable of disrupting laser operation outside a specific range of operating conditions, and have further established that this instability may be suppressed by limiting the growth of the spectral components of the optical field which mediate the instability.